The growing demand for freeze-dried products has led to ever increasing demands for more efficient means of performing all the operations necessary to obtain a freeze-dried product. A critical step in the process is the actual freezing of the product to be dried. Generally it is recognized that rapid freezing will promote the growth of a large number of well ordered dendritic ice crystals of small size and that slow freezing will produce a lesser number of ice crystals of a larger size. Thus, in the dehydration of solid foods, the cellular structure of the food can be modified by varying the freezing rate. Since liquid food materials such as coffee extract lacks the cellular structure of solid foods, various methods have been advanced to promote the formation of large crystals during the freezing process of the extract. A large ice crystal structure when sublimed permits a product which is darker and more coffeelike in color than a product based on the smaller ice crystal structures.
The freezing of coffee extract in preparation for freeze-drying is commonly done on a continuous stainless steel or Teflon coated belt. Liquid, slushed and/or foamed extract is normally fed onto one end of the belt by a pipe or spreader device. Freezing or heat removal is by conduction through the belt such as by continually spraying the underside of the belt freezing surface with brine. Agitation or vibration of the extract may be used to prevent supercooling and spontaneous nucleation and also to make heat removal from the extract more uniform. More commonly, static freezing is employed in which no overt agitation of the extract is used, and the liquid material is slowly frozen by contact with the cooled belt surface. Still another method is to seed the extract with previously formed ice crystals. These methods, however, have inherent disadvantages or limitations. Specifically agitation is only feasible during the early stages of chill-down while the extract is still liquid. Likewise seeding techniques can only be performed while the extract is still in a liquid state.
The freezing rate for coffee extract is a major factor in the final product's color. A high freezing rate produces a product which is light brown or tan in color. As the freezing rate is slowed, a darker, more desirable product is obtained. It has been found, as discloseed in U.S. Pat. No. 3,399,061 to Lutz, that final product color of freeze-dried coffee may be controlled by adjusting the freezing rate of the extract. The most critical portion of the freezing step is the initial period wherein proper ice crystal growth is initiated. The methods of freezing coffee extract employed by the prior art do not permit substantially all the extract to be maintained at a critical temperature for a period of time. Typically a bed of coffee extract 1/4 to 1 inches thick on a freezing surface will have a multiplicity of temperature zones throughout the bed. Additional hold-up time is necessary for substantially all of the extract to equilibrate at a particular temperature using the methods disclosed in the prior art.
Another disadvantage of the prior processes is that when the freezing steps are completed, the coffee extract exists in the form of a frozen slab. Since the desired finished product is granular in form, the extract must be subdivided prior to or after freeze-drying. Subdivision of extract is normally accomplished prior to freeze-drying. Typically, the frozen slab is fed into a mill with a series of rotating blades. On the discharge side of the mill is a screen whose hole size permits only the passage of particles when they are sufficiently reduced in size by the rotating blades. Oversized particles are effectively limited by the screen itself since they will not pass through the selected screen size opening. Unfortunately, undersized particles, including very fine material which is generated by the milling process itself, easily passes through the screen. To remove these undersized particles from the product requires additional processing steps.
It would, therefore, be highly desirable if a simple method were devised for freezing coffee extract which produces a finished product which is dark and coffee-like in color. It is also desirable to eliminate the grinding and fines removal processing steps of conventional freeze-dried coffee processing.
As used in this application "extract" means an aqueous extract of coffee solids having a concentration of between 15% and 50% soluble solids by weight. "Ice point" is that temperature at which water in the extract begins to crystallize into water ice. "Eutectic point" means that temperature at which the specific mixture of coffee solids volatile aromatics and water having the lowest melting point of any other mixture in the extract solidifies. In effect, this temperature is the lowest possible melting point of any material contained in the extract. "Product temperature" as used herein means the temperature of the dried coffee extract. "Freeze drying or lyophilization" as used in this context refers to the process of drying whereby water is removed directly from the solid state to the vapor state without passing through an intermediate liquid state (sublimation). This process also includes that portion of the drying process wherein all the water ice crystals have been sublimed and an eutectic mixture of coffee solids aromatics and water is dried to a stable moisture content. In this portion of the freeze-drying process it is possible that some evaporation of water from the liquid state may occur without appreciable melting of the frozen extract. However, even at this stage of the drying, the product temperature should be kept below the eutectic point of the material dried in order to avoid melting any portion of the frozen extract.